1. Field of the Invention
The present invention generally relates to a flexible electret transducer assembly, and more particularly, to a flexible electret transducer assembly having a membrane made of an electret material. The present invention also relates to a speaker having the flexible electret transducer assembly and a method for fabricating the flexible electret transducer assembly.
2. Description of Related Art
Vision and audition are the most direct sensory responses of human beings. Thus, scientists have been dedicated to developing various renewable vision and audition related systems. Moving coil speaker is still the major product in the market among all the existing renewable speakers. However, along with people's increasing demand to high quality sensory enjoyment and the ever-decreasing sizes of 3C products (Computer, Communication, and Consumer Electronics), speakers which have low power consumption, light weights, and small sizes and are designed according to human factors engineering are to be developed and broadly applied in the near future.
The existing speakers can be categorized into direct and indirect types according to their radiation patterns or can be categorized into moving coil speaker, piezoelectric speaker, and electrostatic speaker according to the driving patterns thereof. The moving coil speaker is currently the most commonly used and most mature product. However, a moving coil speaker cannot be compressed due to the physical structure thereof. Accordingly, moving coil speaker is not suitable for 3C products and home entertainment systems which have their sizes reduced constantly.
A piezoelectric speaker pushes a membrane to produce sounds based on the piezoelectric effect of an electrical material (i.e., the material is deformed when an electric field is supplied thereon). A piezoelectric speaker has a compressed and small structure. Electrostatic speaker is a hi-end earphone or speaker in the current market. According to the operation principle of a conventional electrostatic speaker, a conductive membrane is disposed between two open-hole electrical backplates to form a capacitor. An electric field is produced by supplying a DC bias to the membrane and an AC voltage to the two electrical backplates. The conductive membrane is driven by the electrostatic force generated by the electric field to vibrate and accordingly produce audio. The conventional electrostatic speaker needs a bias of up to hundreds or even thousands voltages, and accordingly a high-cost and bulky amplifier has to be used and which makes the conventional electrostatic speaker very difficult to be promoted.
An electret transducer assembly is disclosed in U.S. Pat. No. 4,249,043. This electret transducer assembly includes a plastic body and a screwed-in central recess. In addition, an electrical backplate having a plurality of protrusions and vent holes is assembled to the body through a screw, wherein the screw threads of the bolt and the nut are connected to each other. In addition, an aluminum-coated electret membrane is extended along an exposed surface of the electrical backplate and is separated from the electrical backplate by the protrusions on the electrical backplate. In this patent, the electrical backplate and the protrusions are formed integrally. Because the material for forming the electrical backplate is highly adhesive and accordingly requires enough room to flow, the electret transducer assembly cannot be made too thin, especially when it is formed integrally with the bolts. Thus, this patent is only suitable for non-flexibly applications.
Audio is a major element in the future applications of flexible electronics. However, flexible electronics has to have the characteristics of softness, thinness, low driving voltage, and high flexibility. Thus, how to fabricate elements having the characteristics of flexible electronics has become a major subject.
The space between the electrical backplate and the membrane in a conventional electrostatic electret transducer assembly is about 0.5 mm. To achieve an electric field of sufficient intensity for driving the membrane, a voltage of hundreds or even thousands voltages is required such that the vibrated membrane can push the air and accordingly produce sounds. Even though the entire thickness of an electret transducer assembly is as thin as a few millimeters, it cannot be applied to a portable product or a product which is rolled up to further reduce the surface area. Moreover, the circuit driver for providing the high voltage usually has a large volume, which also makes it impossible to reduce the size of the electret transducer assembly.
Accordingly, a flexible and small electret transducer assembly is desired, and the fabrication of this assembly should be simple and suitable for mass production.